The ever-increasing packing density of chips requires formation of finer resist patterns with greater precision in lithographic processes. In order to respond to this requirement, development of exposure technologies for attaining higher resolution is being pursued along the lines of using light sources with shorter wavelengths, extending from excimer laser beams such as KrF (248 nm), ArF (193 nm) and F2 (157 nm) up through soft X-rays with wavelengths of 50 nm or shorter (e.g., EUV-ray with a wavelength of 13.5 nm) as the ultimate light source, in place of hitherto used near ultraviolet rays such as g-ray and i-ray.
When soft X-rays (e.g., EUV-ray) are used as light source, lens materials suitable for dioptric light systems hitherto utilized are not obtainable. So it is a foregone conclusion to adopt a catoptric light system and apply thereto a soft X-ray reflection-type reduced projection exposure method using a plurality of multilayer film mirrors and a reflection mask. In this case, there is a defect that the amount of rays finally reaching the wafer surface is considerably decreased by reflection from a plurality of multilayer film mirrors because each mirror has naturally its reflectivity limits. Accordingly, it is required for application of the soft X-ray reflection-type reduced projection exposure method to impart the highest possible sensitivity to a resist used. However, there is a problem that no resist having sensitivity high enough for practical use is found yet.
In addition, materials used for resist have strong absorption in the soft X-ray region. Therefore, there is a problem that the rays cannot reach the bottom of resist and patterns with rectangular profiles cannot be formed. Although it is supposed that reduction in thickness of resist film is a solution to that problem, this solution induces another problem of causing a shortage of dry etching resistance.
Further, with the future trend toward a growing need for finer and more precise resist patterns, a problem of how to control line-edge roughness (abbreviated as “LER”) of resist patterns becomes important.
With the intention of dealing with the problems facing the soft X-ray reduced projection exposure method, various reports have been made, wherein are included the patterning method to which a triple-layer resist method is applied (JP-A-5-121312), the method of using a high concentration of photo-acid generator (JP-A-2002-55457), the patterning method of providing a layer made up of a material generating electrons with efficiency (JP-A-5-136026), the materials using polycyclic unsaturated hydrocarbon derivatives (JP-A-2001-328964), the patterning method of controlling disturbances in pattern profiles by inhibiting the production of low molecular-weight compounds from resist films (JP-A-2000-347411) and the improvements from the apparatus side (JP-A-7-263322 and JP-A-2002-15970).
However, those methods are insufficient to solve the current problems, and new solutions are strongly wanted.